Add space to the end of filter conditions in the query

[alex9smith]

Fixes #6

Previously only the first filter condition in the query was applied. Due
to the order in which filters are constructed, this was usually keyword.

Someone making a search with

f = Filters(
    keyword = "paypal",
    timespan = "7d",
    theme = "TAX_ECON_PRICE"
)

would get the same results as someone using

f = Filters(
    keyword = "paypal",
    timespan = "7d"
)

which is incorrect.

This is because the API needs a space between each of the filter conditions
in the query.

Add a space at the end of every filter condition when constructing the
query string. I have tested and the API returns the same results if a
space is at the end of a query with only a single condition, ie.

f = Filters(
    keyword = "paypal",
    timespan = "7d"
)

and

f = Filters(
    keyword = "paypal ",
    timespan = "7d"
)

have the same results, so it is safe to always add the space in.

Also add a changelog so it's easy to keep track of what's happening when a new version is released.