Update pom.xml

Update pom.xml to specify java 1.7 for building.  by default Maven uses 1.3 unless you specify.
Update POM to use Java 1.5 to compile
Merge pull request #41 from trainerbill/master

Update pom.xml
Merge branch 'stable'
Merge branch 'master' of github.com:paypal/sdk-core-java
add content type header when creating OAuth token
Merge branch 'http-header-fix' into v1.6.6
bump up version
Merge branch 'v1.6.6' into stable