rhbz888090 - Add local Etag caching mechanism.

Pull Command now uses an ETag cache to send to the server, and also is able to handle 304 responses when the Etags match.
rhbz888090 - Enhance ETag client mechanism.

Client now checks for file MD5 checksums to validate the cache is up to date.
Add two parameters to use the cache and to purge the cache.
Merge branch 'rhbz888090' into integration/master

Conflicts:
	pom.xml
	zanata-client-commands/src/main/java/org/zanata/client/commands/pull/PropertiesStrategy.java
	zanata-client-commands/src/main/java/org/zanata/client/commands/pull/PullStrategy.java
	zanata-client-commands/src/main/java/org/zanata/client/commands/pull/UTF8PropertiesStrategy.java
	zanata-client-commands/src/main/java/org/zanata/client/commands/pull/XliffStrategy.java
	zanata-client-commands/src/main/java/org/zanata/client/commands/pull/XmlStrategy.java
rhbz888090 - Fix merge issues.