Improve MOTHUR_FILES and MOTHUR_TOOLS

[mothur-westcott]

Allow multiple paths to be stored as places to look. Something like:

MOTHUR_FILES="/Users/swestcott/Desktop/mothur;/Users/swestcott/Desktop/mothurbugs;/Users/swestcott/Desktop/release"

MOTHUR_TOOLS="/Users/swestcott/Desktop/mothur;/usr/bin;/Users/swestcott/Desktop/mothur/mothurTools;PATH"

[mothur-westcott]

mothur > set.dir(mothurfiles='/Users/swestcott/Desktop/GreenGenesOTUMapping;/Users/swestcott/Desktop/mothurbugs;/Users/swestcott/Desktop/release')
Mothur's directories:
mothurfiles=
/Users/swestcott/Desktop/GreenGenesOTUMapping/
/Users/swestcott/Desktop/mothurbugs/
/Users/swestcott/Desktop/release/

[mothur-westcott]

Changes set.dir 'temp default' to 'mothurfiles' for clarity.

[mothur-westcott]

• Add multiple locations to inputdir option as well

[mothur-westcott]

mothur > set.dir(input="/Users/swestcott/Desktop/release/;/Users/swestcott/Desktop/mothurbugs/;mothurhome")

becomes the following with 'mothurhome' expanded

mothur > set.dir(input="/Users/swestcott/Desktop/release/;/Users/swestcott/Desktop/mothurbugs/;/Users/swestcott/Desktop/mothur/Build/Intermediates.noindex/UninstalledProducts/macosx/")
Mothur's directories:
inputdir=
/Users/swestcott/Desktop/release/
/Users/swestcott/Desktop/mothurbugs/
/Users/swestcott/Desktop/mothur/Build/Intermediates.noindex/UninstalledProducts/macosx/