Cache clearing, update cache scope

[bridwell]

Addresses #44

• Adds methods for manually clearing caches
• Adds method for changing the cache scope of a registered injectable/table/column

[coveralls]

Coverage increased (+0.2%) to 96.813% when pulling 33dd405 on AZMAG:cache_clearing into 28f67c0 on UDST:master.

[bridwell]

To manually clear a cache

# clear an injectable
orca.clear_injectable('my_injectable')

# clear an entire table
orca.clear_table('my_table')

# clear a single column
orca.clear_column('my_table', 'my_col') 

# clear all columns
orca.clear_columns('my_table')

# clear a subset of columns
orca.clear_columns('my_table', ['col1', 'col2'])

To change cache scope:

orca.update_injectable_scope('my_inj', 'iteration')
orca.update_table_scope('my_table', 'step')
orca.update_column_scope('my_table', 'my_column', 'step')

# omitting the scope or passing None turns caching off
orca.update_table_scope('my_table')

[hanase]

Thanks so much for the PR, Scott! This will solve my issue described in #44.

[smmaurer]

Hi @bridwell, the cache management functionality in this PR is fantastic! Sorry for the long delay in merging it.

I'm going to merge it this week and then put out a v1.6 release of Orca -- I'll open a separate issue to discuss the release. cc @hanase @janowicz

Thanks again!