i18n: Allow locating ICU data relative to 'exec_path'.

[srl295]

If the user hasn't otherwise specified a directory,
append NODE_EXEPATH_ICUDIR ( default: ../share/node/icu/ )
and set it as the ICU dir.

If we are in small-icu mode, ALSO actually verify the aforementioned
directory exists (thx libuv) and suppress loading the small-icu data
in that case.

(If we aren't in small-icu mode, there's no reason to incur the cost
of hitting the FS to check whether the directory exists or not at
startup time.)

// cc: @tjfontaine @misterdjules

see srl295/node/pull/22 especialy 12d619f for work in progress

[srl295]

Does the WIP make any sense so far?