Use / or \ in textplot if the function is increasing or decreasing

Before:

0.99845 |                                      ... ....
        |                                    ..        ..
        |                                   .            .
        |                                  .              .
        |                                 .                .
        |                                .                  .
        |                               .                    .
        | .                            .                      .
5.55112 | -.--------------------------.------------------------.-
        |   .                        .                          .
        |    .                      .
        |     .                    .
        |      .                  .
        |       .                .
        |        .              .
        |         .            .
        |          ..        ..
-0.9984 |            ........
          -10                    0                          10

After:

0.99845 |                                      ... ....
        |                                    ..        ..
        |                                   /            \
        |                                  /              \
        |                                 /                \
        |                                /                  \
        |                               /                    \
        | \                            /                      \
5.55112 | -\--------------------------/------------------------\-
        |   \                        /                          \
        |    \                      /
        |     \                    /
        |      \                  /
        |       \                /
        |        \              /
        |         \            /
        |          ..        ..
-0.9984 |            ........
          -10                    0                          10
Change preview to use BytesIO instead of StringIO

This fixes it to work in Python 3. Since BytesIO is available in Python 2.6,
the use of StringIO has been deprecated. See
https://code.google.com/p/sympy/issues/detail?id=3984.

This fixes, in particular, init_printing(use_latex='png') in Python 3.
Don't install isympy as isympy3 in Python 3
Merge branch 'master' into preview_bytesio

Conflicts:
	sympy/interactive/printing.py
Fixed minor dummification issue.

Nothing too big, just looking at the namespaces used for eval rather than the
names of modules passed in.
Remove translation cruft.
Make degree(0) == -oo in the polys

Previously, degree(0) was -1. This makes sense for the dense representation,
because then one can use degree in place of len(f) - 1, but outside of that,
it makes little sense. In particular, the very important formula regarding
degree, deg(f*g) == deg(f) + deg(g), does not hold unless deg(0) == -oo.
Therefore, any code that does arithmetic on a degree and then makes a
comparison against it (like "if degree(f) - 1 > 0"), such as some code in the
Risch algorithm) was previously for 0 but generally correct now.

Fixing this just means being more careful about when you mean degree(f) and
when you mean len(f) - 1 in the dense representation algorithms.  Another
argument for this is that the change in the sparse code required no additional
changes anywhere; apparently the degree does not correspond to any natural
quantity in the sparse representation.
Fix typo
Fix some additional issues caused by degree(0) == -oo
Change the solve flag "minimal" to "particular"

Aside from the name being confusing, it was clobbering the minimal flag that
would otherwise be passed forward to checksol.
Added dummify flag to lambdify.

Replaced logic in lambdify which attempted to determine if dummification should
or should not happen, with a flag (default True) for dummification. The idea is
if someone wants to read the output and are already supplying 'sympy' as a
module, they shouldn't have a problem supplying dummify=False as an argument.
Fix some uses of solve(minimal=True) to solve(particular=True)
Drop unused skip_abs option for Expr.compute_leading_term and calculate_series

    This was introduced in 89c7626 and then this option was unused since 9c46974
Drop obsoleted docs about skip_log option of compute_leading_term()

    This was replaced by logx option in 9c46974d
Add logx option for lseries()
Drop _eval_lseries from exponential.py
Rewrite calculate_series with lseries
Explicitly specify Order variables in various series methods
Remove unneeded substitution of log(w)
Merge branch 'master' into logx-for-lseries