Verify castability before actually doing it

We are experiencing problems with mis-formed lines, where we think a
field is supposed to be a Map, but due to a series of unfortunate events
it is actually a String. When we encounter this line, Hive flips out and
fails the entire job due to a recurring ClassCastException.

This patch simply checks the castability of the variable before actually
doing the cast. It returns NULL if it's not castable, and logs the
toString() of it at warn level.
Merge branch 'release/1.1.5' into develop
Added basic timestamp tracking.
Added check for missing fields.
fixed conflict
wMerge branch 'guyrt-timestamp-support' into feature/25-timestamp-support
Merged timestamp support, plus added support for numeric timestamps
fixes #28 , error after alter
updated readme
gitflow release
gitflow release
Merge branch 'release/1.1.6'
Merge branch 'release/1.1.6' into develop
merged issue #28
fixes issue #25
updated readme
removed static list used to deserialize json struct
updated docs
updated docs
updated docs