Ensure that the environ variable of database location is used through out.

[kindofluke]

Great library!

I know you all haven't worked on this in a little while but I wanted to offer this PR.

In looking at rxnorm_link_run, the db_file location is determined through os.environ.get('SQLITE_FILE')

However, when actually running linkage though rxnorm values are looked up using the RxNormLookup class which hardcodes the database to always be in databases directory.

See below from rxnorm.py:

class RxNormLookup (object):
	""" Class for RxNorm lookup. """
	
	sqlite = None
	cache_drug_class = False		# will be set to true when the prepare_to_cache_classes method gets called
	
	
	def __init__(self):
		absolute = os.path.dirname(os.path.realpath(__file__))
		self.sqlite = SQLite.get(os.path.join(absolute, 'databases/rxnorm.db'))

To fix this, I reused the same code from rxnorm_link_run which checks the environment variable first and happily falls back on the databases location if it is not populated.