spell check
add dimensionless 'units' for unitless conversions

-> convert percent, per mille, ppm, dB, etc
is_valid_unit? tests if a given unit is known
added is_compatible_unit function
added symbol for dBm
fixes for translations

-> accidentally used Conversions[@unit_name] in unit_name_t; in older
   versions, something like [:temperature].to_s would yield
   'temperature', whereas now it yields '[:temperature]', so better use
   Conversions[@unit_name][0]
extend alchemist for pF-value
fix variable name and indent in test
fix tests (to_f for all comparisons)
use short non-breaking space between value and unit
fix and translate psi (pressure)
fix .to_s for unitless values
extend alchemist for RPM, amperes and acceleration
add common metric & imperial units
add standard gravity
Adds ohm to common units
Merge remote-tracking branch 'am/master'