feat(DB): Add method to export clusters to a database file.

[d-bohls]

Fixes #231

• This contribution adheres to CONTRIBUTING.md.
• New tests have been created for any new features or regression tests for bugfixes.
• tox successfully runs, including unit tests and style checks (see CONTRIBUTING.md).

[coveralls]

Coverage decreased (-0.005%) to 68.172% when pulling ae89c0c on d-bohls:addClusterSave into 730305b on ni:master.

[coveralls]

Coverage decreased (-0.005%) to 68.172% when pulling 5218cec on d-bohls:addClusterSave into 1af04b0 on ni:master.

[d-bohls]

nxdb_save_database is documented here:

http://zone.ni.com/reference/en-XX/help/372841P-01/nixnet/nxdbsavedatabase/

I thoroughly tested a ton of scenarios for nxdb_save_database and here is what I found:

1. A database object can only be saved to a filepath with an xml (FIBEX) extension. Trying to save to LDF or DBC errors, even if the original file was one of these.
2. The filepath param is only optional for a database object if the opened database has an xml file extension (a FIBEX database). XNET will error if you open a .dbc or .ldf database and call save without a filepath.
3. Filepath is never optional when you call nxdb_save_database on a cluster.
4. CAN clusters must specify a filepath with a .dbc extension.
5. LIN clusters must specify a filepath with a .ldf extension.

Given these results along with the way the C API documentation talks about exporting a cluster, I felt it more appropriate to:

1. name the method Cluster.export instead of Cluster.save
2. make the filepath parameter required instead of optional.