xls wb creation, new font and cell-style options

These changes arose from my need to create xls workbooks
with an assortment of font and cell-style options.

* upgraded Clojure version to 1.5.1
* now testing against Clojure version 1.6.0
* suppressed reflection warnings during tests
* added __create-xls-workbook__ function
* added following options to __create-font!__
  __:name__        (font family - string)
  __:size__           (font size - integer)
  __:color__          (font color - keyword)
  __:bold__           (true | false)
  __:italic__           (true | false)
  __:underline__   (true | false)

* added following options to __create-cell-style!__
  __:background__    (background colour - keyword)
 __:font__          (font | fontmap of font options)
 __:halign__        (:left | :right | :center)
 __:valign__        (:top | :bottom | :center)
 __:wrap__          (true | false - controls text wrapping)
 __:border-left__   (:thin | :medium | :thick)
 __:border-right__  (:thin | :medium | :thick)
 __:border-top__    (:thin | :medium | :thick)
 __:border-bottom__ (:thin | :medium | :thick)

* added tests for the above options.
* in the case of __create-xls-workbook__, added dedicated namespace __xls-test__.