Removed requirement of access key and secret key

Access key and Secret key needed to be defined in the pom file and when
they were not the pom file would not run. Since the access key and
secret key can be optional I have defined it as optional.

Fixes Upplication#3

pom.xml

@@ -10,7 +10,7 @@
 
     <groupId>com.upplication.maven.plugins</groupId>
     <artifactId>s3-download-maven-plugin</artifactId>
-    <version>1.0.3-SNAPSHOT</version>
+    <version>1.0.4-SNAPSHOT</version>
     <packaging>maven-plugin</packaging>
 
     <name>S3 download maven plugin</name>

readme.md

@@ -10,8 +10,8 @@ Configuration parameters
 |bucketName|The name of the bucket|*yes*| |
 |source|The source amazon s3 file key. Empty to download the whole bucket.|*no*| |
 |destination|The destination file or destination folder. Directories *MUST* end with */*| *yes*| |
-|accessKey|S3 access key | *yes* | if unspecified, uses the Default Provider, falling back to env variables |
-|secretKey|S3 secret key | *yes* | if unspecified, uses the Default Provider, falling back to env variables |
+|accessKey|S3 access key | *no* | if unspecified, uses the Default Provider, falling back to env variables |
+|secretKey|S3 secret key | *no* | if unspecified, uses the Default Provider, falling back to env variables |
 |endpoint|Use a different s3 endpoint| *no* | s3.amazonaws.com |
 
 Example: Download a bucket

src/main/java/com/upplication/maven/plugins/s3/download/S3DownloadMojo.java

@@ -24,13 +24,13 @@ public class S3DownloadMojo extends AbstractMojo {
     /**
      * Access key for S3.
      */
-    @Parameter(property = "s3-download.accessKey", required = true)
+    @Parameter(property = "s3-download.accessKey")
     private String accessKey;
 
     /**
      * Secret key for S3.
      */
-    @Parameter(property = "s3-download.secretKey", required = true)
+    @Parameter(property = "s3-download.secretKey")
     private String secretKey;
 
     /**