Removed misleading comment from checkStringValue().

Removed misleading comment on possibly "multiple values" to be checked from
checkStringValue() method.
Fixed typos in comments and other minor issues.
Added support for new value representation "UR".

Added support for new value representation "Universal Resource Identifier or
Universal Resource Locator (URI/URL)" (UR) that has been introduced only
recently with CP-1324 (Add a new Value Representation for URI/URLs).
Updated data dictionary for DICOM 2014b edition.

Updated data dictionary for the latest edition of the DICOM standard, which
has been released on 2014-09-18. Please note that the VR of some existing
attributes changed from ST or UT to the new VR "UR", which has been added
specifically for Universal Resource Identifiers/Locators (URI/URL).
Added functions to enable/disable post-1993 VRs.

Added global functions that allow for enabling and disabling the value
representations (VRs) that have been introduced after the first edition of
the DICOM standard (1993) with a single call. These new functions make it
easier to maintain the various command line tools that use the corresponding
global flags, e.g. dcmEnableUnlimitedTextVRGeneration.
Enhanced rewriting rules in DcmVR::getValidEVR().

Enhanced rewriting rules in DcmVR::getValidEVR() by mapping post-1993 VRs like
"UT" to "UN" instead of "OB" if possible (i.e. if not disabled as well).
Use new VR "UR" for Pixel Data Provider URL.

Use new VR "UR" (instead of "UT") for the Pixel Data Provider URL (0028,7fe0)
element because otherwise two test cases of the dcmdata module fail. This is
because the VR of this attribute changed with CP-1324 (see commit 79c499a).