Additional data.frame support and tests

[eddelbuettel]

This PR further improves data.frame handling, particularly in the tiledb_array type:

• adds several new 'round-turn' write and re-read tests for data.frames (if TileDB 2.0.* is used)
• adds a few new setters/getters for tiledb_array features
• extends some internals (like Dim()) to deal with ASCII type used in an example
• updates documentation
• renames the testthat files to test-foo.R to permit per-file testing (which failed for test_foo.R)
• updates copyright notices in the license statements to 2017 (the first commit)
• other minor changes like renaming two R file to UpperCase and extended SystemRequirements

The commit dates below are all mushed following a rebase after #121 and #122.