Fix bad English in the labels and data

1. Change "Balance Sheet (Assets Accounts)" to "Balance Sheet (Asset Accounts)" because you don't use the plural of pronouns, and because it is inconsistent with the other three Account Types where singular is used (e.g. "Liability Accounts" rather than "Liabilities Accounts").
DONE

2. Change all "Point of Sales" to "Point of Sale". It should be singular.
Reference: http://www.dict.cc/forum-questions/detail-530040-Inflection-of-2-noun.html
DONE

3. Fix description of the Point of Sale module: Was:
    help="Helps you get the most out of your points of sales with "
             "fast sale encoding, simplified payment mode encoding, "
             "automatic picking lists generation and more."),
    Now:
            help="Helps you get the most out of your Point of Sale with "
                 "fast sale entry, simplified payment entry, "
                 "automatic picking list generation and more."),
DONE

4. Change "Products Categories" to "Product Categories"; the pronoun should be singular.
DONE