Capitalize CacheType enums
Some comments for CacheTypes and small refactoring.
Add a "shark.cache.type" flag in TBLPROPERTIES

This allows the user to pass a string representation of a supported cache type (heap, tachyon).

The flag addition also fixes a bug where passing 'shark.cache'='false' causes an InvalidCacheTypeException to be thrown.
Remove "shark.cache.type" property. Handle "shark.cache"="false".
Merge pull request #148 from harveyfeng/partition-caching

Add a new 'shark.cache.type' table property. Also included are a few comments and some small refactoring.