Moving existing executables into executables directory.
Minor cleanup following the overlapping commits.

1. Removed import of dbtk_tools into dbtk_tools
2. Moved TO DOs out of docstrings and into comments following Code Like a Pythonista
Removing dbtk_from_txt since it has been merged into dbtk_tools.
Added table_info class and changed setup function to create_table; added support for PostgreSQL.
Fixed data type conversion for "skip" and "combine" and enabled script to set number of header lines to skip.
Enabled command line argument specifying database engine; separated command line argument checking procedure from cursor functions into separate function.
Corrected variable name bug that had broken datacleanup.emptyvaltonull
Updated documentation; created separate function for creating database (shouldn't be in create_table); added dbtk_bbs.py.
You can now specify in each DBTK script the name of a cleanup function to be called for each value.
Create_table now accepts an opened file instead of a URL, so that the file can be processed before being passed to the function.
Added docstrings to all functions in dbtk_tools. Minor change to the way length is specified in variables in convert_data_type.
Minor changes to conform with PEP style guide.
Fixed variable length issue in convert_data_type; minor change to the way passwords are entered, since getpass wasn't functioning very well in the windows executables.
Changing cleanup.emptyvaltonull to run on a single line rather than a file.
Split out the skipping of data file headers into a new function dbtk_tools.skip_header that is now called by dbtk_tools.open_url, so that this functionality is now available to any script/function opening a file.
Updated datacleanup.correct_invalid_data to allow for multiple values with no type restrictions. The nullindicators variable of class Table is now a set that can contain any desired values. This also allows correct_invalid_data to perform the job of the now removed datacleanup.emptyvaltonull by including the empty string in the set of nullindicators.
Added database toolkit for the Portal Project - Mammals dataset.
Changed portal mammals toolkit to not automatically generate a primary key, since it already contains one.
Changed ID column on portal mammals table to primary key.
Rough draft of just the counts table for the Breeding Bird Survey. Holy cow, this thing downloads millions of records...