Fix bug in overwrite when using schema-qualified table names

[JoshRosen]

This patch fixes #97, an issue where using SaveMode.Overwrite with schema-qualified table names (such as my_schema.my_table) would lead to "Invalid operation: syntax error at or near ".";" messages from Redshift.

The problem is that the ALTER TABLE ... RENAME command only accepts unqualified table names as the new names, so we need to strip off the schema prior to performing the renaming.

In order to keep this code clean, this patch introduces a new TableName case class which manages all of the escaping and parsing logic for table names, then uses that class to fix the Overwrite issue.

[JoshRosen]

/cc @cfeduke

[JoshRosen]

Argh, it turns out that it's legal for . to appear inside of delimited identifiers, so I'll have to do a quote-aware splitting here.

[JoshRosen]

My experience in working with a wrapper for TableName makes me want to use similar wrappers to automatically handle column name quoting in the other parts of spark-redshift. I'll leave that refactoring to future work, though.

[codecov-io]

Current coverage is 94.73%

Merging #102 into master will decrease coverage by -0.08% as of a9b41a1

@@            master    #102   diff @@
======================================
  Files           11      12     +1
  Stmts          444     475    +31
  Branches       109     116     +7
  Methods          0       0       
======================================
+ Hit            421     450    +29
  Partial          0       0       
- Missed          23      25     +2

Review entire Coverage Diff as of a9b41a1

Powered by Codecov. Updated on successful CI builds.

[marmbrus]

LGTM