v0.2.0

This release includes small bugfixes and new aperture photometry functions, ellipse coefficient functions and a Kron radius function. List of changes:

• [breaking change] theta field in extract() output is now in
  radians rather than degrees, for compatibility with new ellipse
  aperture functions.
• [deprecation] Change mask_ellipse() parameters from ellipse
  coefficients to ellipse axes and position angle, to match aperture
  functions. (Old behavior still works as well.)
• [deprecation] Change apercirc() to sum_circle(), to match
  new aperture functions. (Old name, apercirc(), still works.)
• Add sum_circann(), sum_ellipse(), sum_ellipann(),
  kron_radius(), ellipse_coeffs(), ellipse_axes() functions.
• Exact mode aperture photometery in all functions, with subpix=0.
• Enable variable thresholding in sep.extract. [#11]
• Fix bug in background masking. This bug impacted masking in all
  functions that used masking. Also affected C library.
• More detail in error messages coming from within the C library.
  More helpful error message for non-native byteorder arrays.
• Add ability to change pixel stack size used in extract(), with
  set_extract_pixstack() function